This invention relates to apparatus for generating ungrounded voltages for superposition on a signal-bearing transmission line. In a commonly assigned, copending U.S. application, Ser. No. 275,411, filed July 26, 1972, now U.S. Pat. No. 3,803,355, there is described a circuit arrangement for line monitoring for wire breaks in direct current, keyed data transmission systems. The transmitting and receiving units of a subscriber station are provided with a low impedance internal resistance for the transmitter and a high impedance internal resistance for the receiver. Each internal impedance is connected into the shunt arm of a bridge circuit, having bridge arms composed in each case of the balancing network circuit, two complementary bridge resistances and the line. In each terminal station a first voltage source connected into the line circuit and a second voltage source connected into the balancing network circuit are provided. These voltage sources generate a monitoring current flow on the subscriber line in the presence of inversely-poled sending voltages across the terminal stations.
The arrangement set forth in the aforementioned application permits line monitoring through measurement of the conduction current. Thus, not only can a line break be detected, but also a breakdown in the power supply. The switching in of the two supplementary direct voltages has as a consequence that the transmission line always carries current in the operating state. When a breakdown occurs in the power supply, the station impedance becomes so high that a line break is simulated.
In providing the two supplementary voltage sources, one has to start from the assumption that the construction of prior art DC data transmission systems have low transmitting voltages and do not admit grounding of the two voltages. Therefore, to generate a voltage which is free of earth potential, a principle is employed which is applied in the known telegraph keying circuits. This principle contemplates an AC voltage being inductively balanced-out by a continually operating blocking oscillator; this AC voltage is rectified and supplied to the circuit as a DC voltage, which is independent of earth potential.
An object of this invention is the provision of a simple circuit arrangement for producing the aforementioned two supplementary voltages.